Display device

ABSTRACT

In normally black double cells, the viewing angle (especially for DSTN) is increased by adding a retardation foil, having its optical axis parallel to an optical axis of the analyzer.

[0001] The invention relates to a liquid crystal display devicecomprising, between a polarizer and an analyzer, a first layer oftwisted liquid crystal material having a twisted structure between twotransparent substrates, and a second layer of twisted liquid crystalmaterial having a twisted structure between two transparent substrates,with a twist sense which is opposite to that of the first layer oftwisted liquid crystal material and with a substantially identical twistangle.

[0002] Such double cells are often used in, for example, automotivedisplays. The two cells may have, for example, a common substrate of,for example, glass or a synthetic material. The used liquid crystaleffect is mostly the (super)twisted nematic effect (S)TN, but also otherliquid crystal materials such as discotic materials are applicable.

[0003] An example of such a double cell is shown in, for example, U.S.Pat. No. 5,287,207. Notably for S-TN double cells, the second cellusually functions as a color compensator. Although a satisfactorycontrast is obtained in the case of perpendicular passage of the light,and also for, for example, orange light in the case of oblique passage,it appears to be difficult to manufacture such a double cell in a simplemanner (for example, by adapting the thickness(es) and/or birefringenceof the layer(s)) with a satisfactory contrast upon oblique passage of,for example, green light or white light for which there is a greatdemand in displays for “automotive” applications. The contrast isconsiderably lower for green light at an angle, while leakage of lightthrough non-addressed pixels occurs.

[0004] It is, inter alia, an object of the invention to obviate one ormore of the above-mentioned problems. To this end, a liquid crystaldisplay device according to the invention is characterized in that atleast one optical retardation foil is present between the analyzer andthe polarizer, which retardation foil has an optical principal axiswhich is substantially parallel to one of the optical axes of theanalyzer (transmission or absorption axis).

[0005] A retardation foil is understood to be a self-supporting ornon-self-supporting layer of a birefringent material, or a layer havingan optically compensating or retarding effect (an optically anisotropiclayer). Upon birefringence, the refractive index varies in dependenceupon the direction of the vector of the electric field associated with alight ray. Birefringent material has only one axis for which a light raywith the vector of the electric field along this axis is diffracted atan extraordinary refractive index n_(e). The relevant axis is referredto as the optical principal axis of the material. For light rays withthe vector of the electric field perpendicular to this axis, therefractive index may be the same for all directions (ordinary refractiveindex n_(o)). If the refractive index varies perpendicular to this axis,then a bi-axial material is concerned. In this case, one of the axes,preferably the axis having the largest refractive index, is parallel tothe (transmission) axis of the analyzer.

[0006] Said retardation foil may comprise a plurality of retardationlayers which are jointly present in one position (for example, betweenthe double cell and the analyzer), but these (sub-)layers may bealternatively present at separate locations.

[0007] It is found that the contrast is considerably enhanced due tosaid choice of the retardation foil with a principal axis which issubstantially parallel to the transmission axis of the analyzer(deviation of at most ±5%), notably when a value R of between 100 and400 nm is chosen for the retardation of the foil R (or of the joint(sub-)layers). The invention is notably applicable to STN double cellshaving a positive optical anisotropy (Δn=(n_(e)−n_(o))>0). The inventionis also applicable to double cells in which the second cell (referred toas compensator cell) comprises a twisted liquid crystal material havinga negative optical anisotropy, for example, discotic materials. Thetwist angles are then preferably in a range between 45 and 315 degrees.

[0008] These and other aspects of the invention are apparent from andwill be elucidated with reference to the embodiments describedhereinafter.

[0009] In the drawings:

[0010]FIG. 1 is a cross-section of a display device according to theinvention,

[0011]FIG. 2 is a cross-section of another display device according tothe invention, while

[0012]FIGS. 3 and 4 show isocontrast curves for a conventional doublecell and for a double cell as shown in FIG. 2, to which the measureaccording to the invention has been applied.

[0013] The drawings are diagrammatic and not drawn to scale.Corresponding parts are generally denoted by the same referencenumerals.

[0014] The display device 1 of FIG. 1 comprises a first display cell 10with a layer 11 of liquid crystal material, having a positive dielectricanisotropy and a positive optical anisotropy, between two transparentsupporting plates 2, 3. On the side of the liquid crystal material, thesupporting plates 2, 3 are provided with electrodes 12, 13 defining, forexample, a matrix of pixels. The pixels may be driven via switchingelements and drive electrodes (active drive). In this example, thepixels are defined by overlapping parts of strip-shaped electrodes whichare supplied with selection and data signals (passive drive). The drivevoltages are obtained, for example, by means of a drive circuit 7 whichconverts incoming information 8 into said drive voltages which areapplied to the electrodes 12, 13 via connection lines 15. Layers 14 ofan insulating material serving, in this example, also as orientinglayers, are present on the layers 12, 13. The layers 14 give the liquidcrystal molecules a twist angle Φ₁ at a voltage of 0 volt across theelectrodes 12, 13. The twist angle Φ₁ is between 135° and 360° and is180° in this example.

[0015] The device further comprises a compensating layer, in thisexample a second cell 20, with a second layer 21 of a liquid crystalmaterial, also having, in this example, a positive optical anisotropy,between two transparent supporting plates 3, 4. The supporting plate 3is chosen to be common for both cells 10, 20, but this is not strictlynecessary; however, this simplifies the manufacture in which also thetotal thickness of the double cell remains small. Layers 24 forinsulation and orientation are present on the supporting plates 3, 4.The liquid crystal material 21 and the orienting effect of the layers 24are chosen to be such that the liquid crystal molecules acquire a twistangle of Φ₂, opposite to Φ₁. The cells 10 and 20 are present between apolarizer 6 and an analyzer 5, whose directions of polarization aremutually crossed perpendicularly. The wall orientation of the layers 14,24 is chosen to be such that the director in the center of the cell 10(at a voltage of 0 volt) is perpendicular to the director in the centerof the cell 20.

[0016] Such a double cell has an excellent contrast in the case of aperpendicular viewing direction and viewing directions at a small anglewith respect to the normal. A further advantage is that the temperaturedependence of the cells 10 and 20 is substantially identical so thatthese double cells can be used through a large temperature range. Thisis notably important for “automotive” applications (dashboards, etc.).

[0017] For viewing directions at a larger angle with respect to thenormal, the contrast will be worse due to leakage of light. Notably forgreen light sources and light sources having a wide color spectrum (forexample, white light or combinations of blue and yellow light), thiscannot be solved easily by adapting the electro-optical properties ofthe liquid crystal material, such as the retardation in the compensatorcell.

[0018] According to the invention, this is largely solved by providingthe double cell with an optical retardation foil 30 having its opticalprincipal axis (the slow axis) of the retardation foil substantiallyparallel to the transmission direction of the analyzer 5. Such auniaxial retardation foil having a retardation of 232 nm was added to adouble cell, in which the composite cells 10 and 20 had a thickness of5.7 μm each and a birefringence Δn of 0.16. The slow axis was in theplane of the analyzer 5, i.e. horizontally to the plane of the drawingin FIG. 1 (in the direction of the arrow n_(x); the two other componentsn_(y) and n_(z) are perpendicular to the plane of the drawing andvertical to the plane of the drawing, respectively, in FIG. 1). Thecomponents n_(x), n_(y) and n_(z) define the refractive index (and theoptical axis of the retardation foil 30). In the relevant case (uniaxialfoil or uniaxial retarder) it holds that n_(x)>n_(y)=n_(z). When a greenlight source 35 (λ=560 nm) was used, the following improvement withrespect to the double cell without retardation foil 30 was found. Thepolar angles (angles with respect to the z direction) at which thecontrast had a value of 1:10 as a function of the azimuth angle weremeasured. The leakage of light was also measured. The polar angles weremeasured, above which the leakage of light was more than 2% as afunction of the azimuth angle. This yielded the following results: TABLE1 Azimuth - angle(°) Contrast 0 45 90 135 180 225 270 315 Double 43 3022 33 48 58 51 65 cell Double 49 40 27 40 52 50 58 63 cell with foil

[0019] TABLE 2 Azimuth - angle(°) Brightness 0 45 90 135 180 225 270 315Double cell 38 >70 26 >70 40 >70 35 >70 Double cell 55 >70 38 >70 49 >7053 >70 with foil

[0020] It is apparent from the Tables that a considerable improvement ofcontrast and a reduction of light leakage through a larger range ofangles is obtained. For the retardation R, preferably a value λ/4<R<λ/2is chosen; in practice, this means that R has a value of between 100 and400 nm, which is also dependent on the wavelength(s) of the light source35.

[0021] The retardation foil may be composed of a plurality ofsub-layers. The sub-layers do not need to be present on the same side;for example, the device of FIG. 2 has two sub-layers 30′, 30″ which arepresent near the polarizer and the analyzer, respectively. Thesesub-layers 30′, 30″ may be alternatively present on the substrate 3. Inthe device of FIG. 2, the twist angle Φ₁ of the sub-cell 10 isapproximately 90 degrees. In this example, the second cell 20 has asecond layer 21 of a liquid crystal material having a negative opticalanisotropy. The orienting effect of the layers 24 is chosen to be suchthat the liquid crystal molecules acquire a twist angle Φ₂, opposite toΦ₁. Such a sub-cell 20 is obtained in this example by choosing adiscotic liquid crystalline material for the liquid material. In thisexample, the layer 21 is liquid but may alternatively comprise apolymerized liquid crystalline material. FIGS. 3 and 4 show theisocontrast curves for a double cell in accordance with the state of theart (Φ₁=Φ₂=90°, d.Δn=430 nm, twisted nematic material for bothsub-cells) and for the double cell of FIG. 2, in which the retardationfoil with a retardation of 220 nm is directly placed behind the analyzerwith the optical axis parallel to the direction of the analyzer. It isapparent from the Figures that the contrast range, limited by contrast1:10, now horizontally covers the angles between −55° and +61° (insteadof between −38° and +42° in FIG. 3) and vertically the angles between−58° and +45° (instead of between −36° and +38° in FIG. 3). The measurethus yields a considerable improvement of the contrast at obliqueangles.

[0022] The invention is of course not limited to the examples shown, butmany variations are possible. For example, the viewing angle of lightsources other than green ones was also improved. For the optical axis ofthe retardation foil, it is not necessary that n_(x)>n_(y)=n_(z), butn_(x)>n_(z)>n_(y) is also satisfactory. The optical axis of this type ofbiaxial foil may extend both along the transmission axis and along theabsorption axis of the analyzer.

[0023] In summary, the invention provides an improvement of the viewingangle in “normally black” double cells in that a retardation foil withits optical principal axis parallel to the analyzer is added.

[0024] The invention resides in each and every inventive characteristicfeature and each and every combination of characteristic features.

1. A liquid crystal display device comprising, between a polarizer andan analyzer, a first layer of twisted liquid crystal material having atwisted structure between two transparent substrates and a second layerof twisted liquid crystal material having a twisted structure betweentwo transparent substrates, with a twist sense which is opposite to thatof the first layer of twisted liquid crystal material and with asubstantially identical twist angle, characterized in that at least oneoptical retardation foil is present between the analyzer and thepolarizer, which retardation foil has an optical principal axis which issubstantially parallel to one of the optical axes of the analyzer.
 2. Aliquid crystal display device as claimed in claim 1, characterized inthat the retardation foil is uniaxial.
 3. A liquid crystal displaydevice as claimed in claim 1, characterized in that a plurality ofretardation layers is present between the analyzer and the polarizer,with the optical principal axis of the combined retardation layers beingsubstantially parallel to one of the optical axes of the analyzer.
 4. Aliquid crystal display device as claimed in claim 1 or 2, characterizedin that the liquid crystal material is nematic and the twist angle is135°-360°.
 5. A liquid crystal display device as claimed in claim 1 or2, characterized in that the joint retardation of the retardation layersis 100-400 nm.
 6. A liquid crystal display device as claimed in claim 1or 2, characterized in that the display device comprises a light sourceemitting substantially green light.
 7. A liquid crystal display deviceas claimed in claim 1 or 2, characterized in that the second layer oftwisted liquid crystal material has a negative optical anisotropy.
 8. Aliquid crystal display device as claimed in claim 7, characterized inthat the twist angle is 45°-135°.
 9. A liquid crystal display device asclaimed in claim 7, characterized in that the second layer of twistedliquid crystal material comprises a discotic material.
 10. A liquidcrystal display device as claimed in claim 7, characterized in that thesecond layer of twisted liquid crystal material comprises a polymerizedliquid crystal material.